1) Field of the Invention
The present invention relates to a technology to perform data transfer between a data transfer apparatus and a target device such as a memory card, particularly, when the data transfer apparatus and the target device are situated away from each other.
2) Description of the Related Art
A conventional technique of this type has been disclosed in Japanese Patent Application Laid-Open Publication No. H5-108563. What is disclosed in this literature is a common clock bus that supplies a clock signal from a host device of an electronic apparatus to plural subordinate devices and a backward path common data bus that transfers a data signal output by each subordinate device, which has received a data signal sent by the host device, to the host device in synchronization with the clock signal. The backward path common data bus is extended from the subordinate device, which is located at a most remote end from the host device, among the subordinate devices connected to the backward path common data bus to directly connect the subordinate device to the host device, whereby a delay time of a data signal from each subordinate device with respect to the clock signal is fixed without depending on a position of the subordinate device.
However, the data transfer between the host device and the subordinate devices in the above literature is one-way transfer rather than two-way transfer. Transfer from the host device to the subordinate devices and transfer from the subordinate devices to the host device are performed via separate circuits, respectively. Therefore, wiring, numbers of both bus drive circuits and bus reception circuits corresponding to the transfer are required. This inevitably results in not only an increase wiring and the numbers of both bus drive circuits and the bus reception circuits.
When the host device and the subordinate devices are separated from each other, in other words, when the subordinate devices are provided at a distance from the host device, a CLK and a command are transferred to the subordinate devices with a delay in time equivalent to the distance between them. In addition, when the subordinate devices return responses to the host device, the responses arrive at the host device with a further delay in time. If the distance is small, the delay is not significant. However, if the distance is long, the delay becomes significant and sometimes the host device cannot receive responses from the subordinate devices, and transfer failure occurs.